Effect of time of application on nitrogen uptake, partitioning, and remobilization in walnut trees

Walnut tree requires a relatively high amount of nitrogen (N). To avoid loss in the environment, N uptake efficiency (NUE) should be optimized. The aims of this study were to evaluate the effect of time of N application on NUE, partitioning, and remobilization in walnut trees. Two-year-old trees were planted in 40-L pots and fertilized with 1 g of 15N-enriched (5 atom %) N at: 1) bud burst, 2) pistillate flower maturity, and 3) late summer. One week after fertilization, the percentage of N derived from fertilizer and NUE were higher in trees fertilized in late summer, than other timings. N uptake was linearly related to root dry weight. At May 2008 harvest, the N stored in trunk and twigs was remobilized to the developing leaves and to the roots. Late summer N application appeared to be the most effective in providing N for walnut spring new growth.